October 14, 1999
This is the initial release of the CSPICE edition of the SPK "Required Reading" document. The underlying Fortran NAIF Toolkit version is N0050.
The SPICE system has been extended to include CSPICE---a version of the NAIF
Toolkit software coded in the ANSI C programming language.
To better accommodate CSPICE users, the Fortran SPK Required Reading document has been translated into this "CSPICE edition." This document closely parallels the current Fortran edition but contains C versions of illustrations of source code, including listings of calling sequences and example programs.
The CSPICE library---the C counterpart of SPICELIB---consists of two categories of functions:
_cFunctions created by f2c have names ending with the suffix
_In this first version of CSPICE, wrappers are provided for the principal SPK application interface functions. Hand-coded versions of other, less frequently used interface functions will be supplied in a later version of CSPICE. The remaining, lower-level functions will not be provided with wrappers.
The set of currently available CSPICE SPK wrapper functions is:
spk14a_c spk14b_c spk14e_c spkapo_c spkapp_c spkcls_c spkez_c spkezp_c spkezr_c spkgeo_c spkgps_c spklef_c spkopa_c spkopn_c spkpds_c spkpos_c spkssb_c spksub_c spkuds_c spkuef_c spkw02_c spkw03_c spkw05_c spkw08_c spkw09_c spkw10_c spkw15_c spkw17_cLess frequently used application interface-level functions referenced here but for which wrappers are not yet implemented are:
frmchg_c frmnam_c namfrm_c spkpv_c spkpvn_c spksfs_c Portions of the DAF family of functions The SPC family of functions
Because of the substantial changes made in this revision of the Fortran
edition of the SPK Required Reading document, the description of those
changes is included here.
When the SPK system was introduced, states of objects (positions and velocities) were stored relative to inertial frames and retrieved relative to inertial frames. Beginning with version 41 of the NAIF Toolkit, states can be stored relative to both inertial and non-inertial frames. Moreover, states may be retrieved relative to both inertial and non-inertial frames. Non-inertial frames may be tied to the rotation of a planet, the orientation of some structure on a spacecraft, an Earth based telescope, etc. By expanding the SPK system in this way, computation that previously required dozens lines of code may now be reduced to three or four lines of code.
This version of the ``SPK Required Reading'' documents for the first time this important expansion of the SPK system.
Also in this version, we document:
The purpose of this document is to describe the NAIF Toolkit software
provided in the software library CSPICE, (C SPICE library) used for producing
and accessing SPICE ephemeris data. In addition this document describes
SPK---the common file format for NAIF's S-kernel and ephemeris portion of the
P-kernel.
This document is intended for all users of SPK (ephemeris) kernel files.
All references are to NAIF documents. The notation [Dn] refers to NAIF
document number.
We'll discuss things in more detail in a moment but in case you are just
looking for the right name of the function to perform some ephemeris task,
here is a categorization of the most frequently used SPK and related
functions in CSPICE. Input arguments are given in lower case and enclosed in
``angle brackets.'' Output arguments are given in plain lower case.
Loading/Unloading an SPK file
spklef_c ( <file>, &handle ) spkuef_c ( <handle> )Retrieving states (position and velocity) using names of objects
spkezr_c ( <object>, <et>, <frame>, <corr>,
<observer>, state, < )
Retrieving positions using names of objects
spkpos_c ( <object>, <et>, <frame>, <corr>,
<observer>, postn, < )
Retrieving states using NAIF ID codes
spkez_c ( <obj_id>, <et>, <frame>, <corr>,
<obj_id>, state, < )
spkgeo_c ( <obj_id>, <et>, <frame>,
<obj_id>, state, < )
Retrieving positions using NAIF ID codes
spkezp_c ( <obj_id>, <et>, <frame>, <corr>,
<obj_id>, postn, < )
spkgps_c ( <obj_id>, <et>, <frame>,
<obj_id>, postn, < )
Calculating ``Uplink and Downlink'' Light Time
ltime_c ( <etobs>, <obs_id>, <dir>, <targ_id>, &ettarLoading/Unloading Binary PCK files (see PCK Required Reading)
pcklof_c ( <binary_pck>, &handle ) pckuof_c ( <handle> )Loading Text based kernels---PCK, SCLK, etc.
ldpool_c ( <text_kernel> )Loading/Unloading C-kernels (see CK Required Reading)
cklpf_c ( <c-kernel>, &handle ) ckupf_c ( <handle> )
The functions listed in this section are the real ``work horses'' of the SPK
and related systems. Not all of the functions in this section are described
in this document. In those cases, the appropriate SPICE document is cited.
Selecting files and segments
spksfs_c ( <target>, <et>, &handle, descr, ident, &found )Computing states from segment descriptors
spkpvn_c ( <handle>, <descr>, <et>, ref, state, ¢er )Correcting for stellar aberration
stelab_c ( pobj, vobs, appobj )Translating between object names and object ID codes (see NAIF_IDS Required Reading)
bodn2c_c ( <name>, &idcode, &found ) bodc2n_c ( <idcode>, name, &found )Translating between frame names and frame ID codes (see Frames Required Reading)
frmnam_c ( <idcode>, name ) namfrm_c ( <name>, &idcode )State transformation matrices (see Frames Required Reading)
frmchg_c ( <from_idcode>, <to_idcode>, <et>, mat6x6 )Classifying frames (see Frames Required Reading)
frinfo_c ( <idcode>, ¢er, &class, &clssid, &found )
Examining SPK files
brief commnt spacitConverting to and from transfer format
spacit tobin toxfr
To help fully understand the science data returned from a spacecraft's
instruments it is necessary to know, at any given epoch, the positions and
possibly the velocities of the spacecraft and all the target bodies of
interest. The purpose of the SPK---which stands for S(pacecraft) and P(lanet)
Kernel---file is to allow ephemerides for any collection of solar system
bodies to be combined under a common file format, and accessed by a common
set of functions.
Historically, ephemerides for spacecraft have been organized differently from those for planets and satellites. They are usually generated through different processes and using different representations. However, there is no essential reason for keeping them separate. A spacecraft, planet, satellite, comet, or asteroid has a position and velocity relative to some center of mass and reference frame. Consequently all of these objects can be represented in an SPK file.
Consider the Galileo mission. Some of the objects of special interest to the Galileo mission are:
Galileo Spacecraft Galileo Probe Earth Moon Earth Moon Barycenter Venus Sun Solar System Barycenter (S.S.B.) Asteroid Ida Ida's Satellite Dactyl Asteroid Gaspra Comet Shoemaker-Levy Jupiter System Barycenter (J.B.) Jupiter Io Ganymede Europa Callysto Goldstone Tracking Station.Each of these objects has a position and velocity (state) relative to some other object. The graph below illustrates which objects will be used as reference objects for representing the states of others.
+Gll
/ probe
/ | o Comet
Gaspra / Gll+ / Shoemaker Levy
Gll +--o / \ /
| / Venus Jupiter o--probe
| / o--+ |
Gll + | / / Gll | Io
| | / / | o-----+Gll
| |/ / J.B.| /
Ida o-------o------o------------------o ----o------+Gll
/ Sun S.S.B. / \ Europa
o \ Ganymede / \
Dactyl \ o \
\ | o Callisto
Earth-Moon Barycenter o----o + |
| Moon Gll |
| + Gll
o Earth
/ \
/ \
/ + Gll
o
Goldstone
This graph is somewhat complicated. Nevertheless, the complete ephemeris
history for all of these objects can be captured in a single SPK file.
(Although we can store the entire ephemeris history illustrated above in a single SPK file, for the sake of data management a project is likely to use several SPK files. However, even in this case, all of the SPK files can be used simultaneously.)
The SPK format is supported by a collection of functions that are part of the CSPICE library---the major component of the NAIF Toolkit. This family of SPK functions provides the following capabilities:
SPK files are binary files. The format of these binary files is based upon a
more abstract file architecture called Double precision Array File (DAF).
Several other SPICE kernels are also based on the DAF architecture. If you
are only going to be a consumer of SPK files or if you will be using a SPICE
utility program for creating SPK files, you can safely ignore aspects of the
DAF system that are not covered by this document. On the other hand, if you
plan to write software for creating SPK files you will probably need to
familiarize yourself with the DAF software contained in CSPICE. The DAF
architecture and supporting software is discussed in [169]. The particular
aspects of the DAF architecture that are relevant to the SPK format are
discussed later in this document (see below---SPK Format).
A binary file suitable for use on one computer, may not be suitable for use
on another computer. For example, SPK files created on a Sun Sparc-10 are not
suitable for use on a PC running SPICE software compiled with the Borland
Bcc32i C++/C compiler. As a result you can't always perform a ``binary copy''
of an SPK file from one machine to another. NAIF provides two utility
programs---TOXFR and SPACIT for converting SPICE binary kernels to a
``transfer format'' that is suitable for text copying from one computer to
another. Once the transfer format file has been copied, the NAIF utilities
TOBIN and SPACIT are available for converting the transfer format file to the
binary format suitable for the new machine.
The utilities TOXFR and TOBIN are ``command line'' programs. To convert a binary kernel to transfer format you simply type TOXFR followed by the name of the binary kernel at your terminal prompt.
prompt> toxfr spk_fileTo convert a transfer format to binary format, you type TOBIN followed by the name of the transfer format kernel.
prompt> tobin transfer_fileThe utility SPACIT is an interactive program that allows you to select an action to perform on a file from a list of possibilities. It can be used to convert to or from transfer format files.
Note that transfer format files are suitable only for moving data from one machine to another. They cannot be ``loaded'' into a SPICE based program to retrieve ephemeris data. Only binary format files can be used for retrieving ephemeris data with SPICE software.
In most environments where SPICELIB is supported, CSPICE can use the same
binary kernels as does SPICELIB. Environments where this local compatibility
exists are:
Sun, Solaris; Sun Fortran; Sun C or Gnu gcc Silicon Graphics, IRIX OS; Silicon Graphics Fortran; Gnu gcc HP Series 700 computers, HP-UX 9000/750; FORTRAN/9000; HP C VAX/VMS; VAX FORTRAN; VAX C PC, DOS/Win95/NT; MS Powerstation Fortran; Borland Bcc32i C++/C NeXT, Mach; Absoft Fortran; Gnu gcc Macintosh??? Mac-ppc???Porting between any pair of supported Fortran and C environments is always possible via binary/transfer conversion. The process is identical to that described above for porting between incompatible Fortran environments.
Since SPK files are binary files, you can't just open them with your favorite
text editor to determine which ephemeris objects are represented in the file.
Instead you need to use one of the NAIF utility programs that allow you to
summarize the ephemeris contents of an SPK file. The first of these is SPACIT
which was introduced above. The second is the command line utility BRIEF.
BRIEF gives a quick summary of the contents of the file and supports a wide set of summary options. SPACIT on the other hand, provides summaries that are more detailed and reflect closely the actual internal structure of the file. Unless you need the more detailed summary, you'll probably find BRIEF to be a better tool for examining the contents of an SPK file.
SPICE kernels may contain ``meta'' data that describe the contents, intended
use, accuracy, etc. of the kernel. This meta data is called the ``comments''
portion of the kernel. Many SPK files contain comments that can help you
decide upon the suitability of the kernel for your application. Two SPICE
utilities are available for examining the comments of a binary
kernel---COMMNT and SPACIT.
We've already introduced SPACIT. COMMNT is similar to SPACIT in that it too is an interactive program. However, COMMNT also allows you to modify the comments of an SPK file. Using COMMNT you can delete the comments of an SPK file, extract the comments to a text file, or append the text from some text file to the comments already present in the kernel.
If you create SPK files, we strongly recommend that you add comments to the kernel that describe who created it, expected usage of the kernel, and the expected accuracy of the position/velocity information contained in the kernel. A comment template is provided in the appendix ``COMMENTS''.
Warning: If you add comments to an SPK (or other binary kernel) using COMMNT, you must wait for the program to complete the task before exiting the program. Failure to wait for COMMNT to finish its work will result in irreparable corruption of the binary kernel. (See the COMMNT User's Guide [212] for details on the use of COMMNT).
Throughout this document we shall be using terms such as reference frame,
state, ephemeris time, etc. We include a brief review of these terms below.
CSPICE contains a family of functions that are designed specifically for use
with SPK files. The name of each function begins with the letters `spk',
followed by a two- or three-character mnemonic. For example, the function
that returns the state of one body with respect to another is named spkezr_c,
pronounced `S-P-K-easier'. A complete list of mnemonics, translations, and
calling sequences can be found at the end of this document.
Each function is prefaced by a complete CSPICE header, which describes inputs, outputs, restrictions, and exceptions, discusses the context in which the function can be used, and shows typical examples of its use. Any discussion of the functions in this document is intended as an introduction: the final documentation for any function is its header.
Whenever an SPK function appears in an example, the translation of the mnemonic part of its name will appear to the right of the reference, in braces. We also continue with the convention of distinguishing between input and output arguments by enclosing input arguments in angle brackets. For example,
spklef_c ( <file>, &handle ); { Load ephemeris file }
All C functions, including those whose names do not begin with `SPK', are
from CSPICE or the standard ANSI C library.
SPK readers are available to perform the following functions.
spkezr_c is the most powerful of the SPK readers. It determines the apparent,
true, or geometric state of one body (the target) as seen by a second body
(the observer) relative to a user specified reference frame.
spkezr_c ( <targ>, <et>, <frame>,
<aberr>, <obs>,
state, < ); { Easier state }
The function accepts five inputs---target body, epoch, reference frame,
aberration correction type, and observing body---and returns two
outputs---state of the target body as seen from the observing body, and
one-way light-time from the target body to the observing body.
The target body, observing body and frame are identified by strings that contain the names of these items. For example, to determine the state of Triton as seen from the Voyager-2 spacecraft relative to the J2000 reference frame
spkezr_c ( "triton", et, "j2000", aberr,
"voyager-2", state, < ); { Easier state }
By definition, the ephemerides in SPK files are continuous: the user can
obtain states at any epoch within the interval of coverage. Epochs are always
specified in ephemeris seconds past the epoch of the J2000 reference system
(Julian Ephemeris Date 2451545.0 ) For example, to determine the state of
Triton as seen from Voyager-2 at Julian Ephemeris Date 2447751.8293,
et = ( 2447751.8293d0 - j2000() ) * spd_c();
spkezr_c ( "triton", et, "j2000", <aberr>,
"voyager-2", state, < ); { Easier state }
where the function J2000 returns the epoch of the J2000 frame (Julian
Ephemeris Date 2451545.0) and the function SPD returns the number of seconds
per Julian day (86400.0).
The ephemeris data in an SPK file may be referenced to a number of different reference frames. States returned by spkezr_c do not have to be referenced to any of these ``native'' frames. The user can specify that states are to be returned in any of the frames recognized by the frame subsystem. For example, to determine the state of Triton as seen from Voyager-2, referenced to the J2000 ecliptic reference frame,
spkezr_c ( "triton", et, "eclipj2000",
aberr, "voyager-2", state,
< ); { Easier state }
spkezr_c returns apparent, true, or geometric states depending on the value
of the aberration correction type flag aberr.
Apparent states are corrected for planetary aberration, which is the composite of the apparent angular displacement produced by motion of the observer (stellar aberration) and the actual motion of the target body (correction for light-time). True states are corrected for light-time only. Geometric states are uncorrected.
Instead of using the potentially confusing terms `true' and `geometric' to specify the type of state to be returned, spkezr_c requires the specific corrections to be named. To compute apparent states, specify correction for both light-time and stellar aberration: `LT+S'. To compute true states, specify correction for light-time only: `LT'. To compute geometric states, specify no correction: `NONE'.
In all cases, the one-way light-time from the target to the observer is returned along with the state.
The light time corrected states returned by the SPK system are simply the
6-vector difference
TARGET_SSB ( ET - LT ) - OBSERVER_SSB ( ET )where TARGET_SSB and OBSERVER_SSB give the position of the target and observer relative to the solar system barycenter. LT is the unique number that satisfies:
| TARGET_SSB ( ET - LT ) - OBSERVER_SSB ( ET ) |
LT = --------------------------------------------------
Speed of Light
Where
| STATE |refers to the length of the position component of a state vector.
TARGET_SSB ( ET - LT ) and OBSERVER_SSB ( ET )
LT_0 = 0
| TARGET_SSB ( ET - LT_(i-1) ) - OBSERVER_SSB ( ET ) |
LT_i = ------------------------------------------------------
Speed of Light
(for i = 1, 2, 3 ... )
It can be shown that the sequence LT_0, LT_1, LT_2, ... converges to LT
geometrically. Moreover, it can be shown that the difference between LT_i and
LT satisfies the following inequality.
i+1
| LT - LT_i | < LT * ( V/C )
where V is the speed of the target body with respect to the solar system
barycenter and C is the speed of light. Let's examine the error we make if we
use LT_1 as an approximation for LT.
For nearly all objects in the solar system V is less than 60 km/sec. The value of C is 300000 km/sec. Thus the one iteration solution for LT has a potential relative error of not more than 4*10D-8. This is a potential light time error of approximately 2*10D-5 seconds per astronomical unit of distance separating the observer and target. Thus as long as the observer and target are separated by less than 50 Astronomical Units the error in the light time returned using option 'LT' is less than 1 millisecond.
For this reason, we use LT_1 to approximate LT when you request a light time corrected state by setting the aberration correction argument in spkezr_c to `LT' or `LT+S'.
You can make spkezr_c perform a better approximation to LT by requesting that it compute a ``converged Newtonian'' value for LT. To do this set the aberration correction to `CN' or `CN+S'. spkezr_c will then return LT_3 as the approximation for light time. The maximum error in LT_3 is less than a nanosecond for any observer/target pair in the solar system.
However, you should note that this is a purely Newtonian approximation to the light time. To model the actual light time between target and observer one must take into account effects due to General relativity. These may be as high as a few hundredths of a millisecond for some objects.
The functions in the SPK family do not attempt to perform either general or special relativistic corrections in computing the various aberration corrections. For many applications relativistic corrections are not worth the expense of added computation cycles. If, however, your application requires these additional corrections we suggest you consult the astronomical almanac (page B36) for a discussion of how to carry out these corrections.
When we observe a distant object, we don't see it as it is at the moment of
observation. We see it as it was when the photons we have sensed were emitted
by or reflected from the object. Thus when we look at Mars through a
telescope, we see it not as it is now, but rather as it was one
``light-time'' ago. This is true not only for the position of Mars, but for
its orientation as well.
Suppose that a large balloon has been launched into the Martian atmosphere and we want to determine the Mars bodyfixed state of the balloon as seen from Earth at the epoch ET. We need to determine both the light time corrected position of the balloon, and the light time corrected orientation of Mars. To do this we compute two light times. The light time to the center of the Mars bodyfixed frame (i.e. the center of Mars) and the light time to the balloon. Call the light time to the center of the Mars frame LT_F and call the light time to the balloon LT_T. The light time corrected state of the balloon relative to the Mars bodyfixed frame is the location of the balloon at ET - LT_T in the bodyfixed frame of Mars as oriented at ET - LT_F.
spkezr_c carries out all of these computations automatically. In this case the computation would be computed by a function call similar to this:
spkezr_c ( "mars_balloon", <et>, "iau_mars", "lt",
"earth", state, < );
spkezr_c uses the following rules when computing states.
Here we illustrate how you could use spkezr_c together with other CSPICE
functions to determine if at a particular epoch ET the Mars Global Surveyor
spacecraft is occulted by Mars.
We will need the lengths of the axes of the triaxial ellipsoid that is used to model the surface of Mars. The CSPICE function bodvar_c will retrieve this information from a loaded PCK file. Note that bodvar_c uses the NAIF ID code for Mars (499) to retrieve the lengths of the axes.
bodvar_c ( 499, "RADII", &nvals, axes ); a = axes[0]; b = axes[1]; c = axes[2];Next we compute the state of Mars relative to Earth and the state of MGS relative to Earth in the Mars bodyfixed frame.
spkezr_c ( "mars", et, "iau_mars", "lt+s",
"earth", marsst, < );
spkezr_c ( "mgs", et, "iau_mars", "lt+s",
"earth", mgsst, < ); {Easier State}
Compute the apparent position of the Earth relative to Mars in the apparent
Mars bodyfixed frame. This means simply negating the components of marsst.
The CSPICE function vminus_c carries out this task.
vminus_c ( marsst, estate );Determine if the line of sight from Earth to MGS intersects the surface of Mars. The CSPICE function surfpt_c will find this intersection point if it exists.
surfpt_c ( estate, mgsst, a, b, c, point, &found );Finally, if a point of intersection was found, was it between the Earth and the MGS spacecraft. To find out we can compare the distances between the intersection point and the spacecraft. The CSPICE function vnorm_c computes the length of the vector from Earth to MGS. The function vdist_c computes the distance between the point and the Earth.
if ( found )
{
betwn = ( vdist_c ( estate, point ) < vnorm_c ( mgsst ) );
}
else
{
betwn = SPICEFALSE;
}
if ( betwn )
{
printf ( "mgs is behind mars\n" );
}
else
{
printf ( "mgs is not behind mars\n" );
}
Low level SPK software uses integer codes to identify ephemeris objects,
reference frames and data representation, etc. At low levels of the SPICE
system only integer codes are used to communicate information about objects.
To some extent, these codes are a historical artifact in the design of the
SPICE system. Nevertheless, these integer codes provide economies in the
development of SPICE software.
High-level SPICE software uses names (character strings) to refer to the various SPICE objects and translates between names and integer codes. Thus to some extent you can disregard the integer codes used by the SPICE internals. However, occasionally, due to the introduction of new ephemeris objects, the name translation software will be unable to find a name associated with an ID code. To retrieve states for such an object you will need to use the integer code for the object in question. If you are using spkezr_c, you can supply this integer code as a quoted string. For example the following two function calls will both return the state of TRITON as seen from Voyager-2. (The NAIF integer code for TRITON is 801; the NAIF integer code for Voyager 2 is -32).
spkezr_c ( "triton", et, "j2000eclip",
aberr, "voyager-2", state,
< ); { Easier state }
spkezr_c ( "801", et, "j2000eclip",
aberr, "-32", state,
< ); { Easier state }
Consult the NAIF IDS Required Reading file for the current list of body codes
recognized by the NAIF Toolkit software.
spkezr_c relies upon two lower level functions that may be useful under
certain circumstances.
The function spkez_c performs the same functions as spkezr_c. The only difference is the means by which objects are specified. spkez_c requires that the target and observing bodies be specified using the NAIF integer ID codes for those bodies.
spkez_c ( <targ_id>, <et>, <frame>, <corr>, <obj_i
state, < ); { SPK Easy }
The NAIF-ID codes for ephemeris objects are listed in the NAIF_IDS required
reading file.
spkez_c is useful in those situations when you ID codes for objects stored as integers. There is also a modest efficiency gain when using integer ID codes instead of character strings to specify targets and observers.
The function spkgeo_c returns only geometric (uncorrected) states. The following two function calls are equivalent.
spkez_c ( <targ_id>, <et>, <frame>, "none",
<obj_id>, state, < ); {SPK Easy}
spkgeo_c ( <targ_id>, <et>, <frame>, <obj_id>,
state, < ); {SPK Geometric }
spkgeo_c involves slightly less overhead than does spkez_c and thus may be
marginally faster than calling spkez_c.
Note that spkezr_c, spkez_c and spkgeo_c do not require the name of an SPK
file as input. These functions rely on a second function, spklef_c, to
maintain a database of ephemeris files. Your application program indicates
which files are to be used by passing their names to spklef_c.
for ( i = 0; i < N; i++ )
{
spklef_c ( ephem[I], &handle[i] ); { Load ephemeris file }
}
spklef_c returns a DAF file handle for each file, which may be used to access
the file directly using DAF functions. Once an SPK file has been loaded, it
may be accessed by spkezr_c.
In general, a state returned by spkezr_c is built from several more primitive states. Consider the following diagram, which shows some of the states that might be needed to determine the state of the Galileo spacecraft as seen from Earth:
Jupiter_Barycenter --- Europa
/ \
/ \
/ Spacecraft
/
/
/
/
SSB
\
\
\
EMB --- Earth
(SSB and EMB are the solar system and Earth-Moon barycenters.)
Each state is computed from a distinct segment. The segments may reside in a single SPK file, or may be contained in as many as five separate files. For example, the segments needed to compute the Earth-spacecraft state shown above might come from the following set of files:
spklef_c ( "barycenters.bsp", &h[0] ); { Load ephemeris file }
spklef_c ( "planet-centers.bsp", &h[1] ); { Load ephemeris file }
spklef_c ( "satellites.bsp", &h[2] ); { Load ephemeris file }
spklef_c ( "spacecraft.bsp", &h[3] ); { Load ephemeris file }
or from the following set:
spklef_c ( "earth.bsp", &h[0] ); { Load ephemeris file }
spklef_c ( "jupiter.bsp", &h[1] ); { Load ephemeris file }
spklef_c ( "spacecraft.bsp", &h[2] ); { Load ephemeris file }
An SPK file may contain any number of segments. A single file may contain
overlapping segments: segments containing data for the same body over a
common interval. When this happens, the latest segment in a file supersedes
any competing segments earlier in the file. Similarly, the latest file loaded
supersedes any earlier files. In effect, several loaded files become
equivalent to one large file.
The number of SPK files that may be open at any one time is limited. For
example, some operating systems limit the total number of files that may be
open at one time to 20. Consequently, your application program may need to
unload some SPK files to make room for others. An SPK file may be unloaded by
supplying its handle to function spkuef_c. The sequence of statements shown
below,
spklef_c ( "file.a", &ha ); { Load ephemeris file }
spklef_c ( "file.b", &hb ); { Load ephemeris file }
spklef_c ( "file.c", &hc ); { Load ephemeris file }
spkuef_c ( hb ); { Unload ephemeris file }
spklef_c ( "file.d", &hd ); { Load ephemeris file }
spkuef_c ( hc ); { Unload ephemeris file }
is equivalent to the following (shorter) sequence:
spklef_c ( "file.a", &ha ); { Load ephemeris file }
spklef_c ( "file.d", &hd ); { Load ephemeris file }
Prior to the inclusion of non-inertial frames in the SPK system, the states
of objects computed by the SPK system required only that you load the correct
SPK files and call the correct functions. The inertial frame transformations
needed for converting from one inertial frame to another are ``hard wired''
into the SPICE system. The transformations are part of the object code of the
CSPICE library---no additional data need be supplied to compute these
transformations. This approach to carrying out inertial frame transformations
was chosen because the transformations are compactly represented and do not
change as the result of further observations. They are essentially
definitions.
On the other hand, the orientation of non-inertial frames with respect to other frames are almost always the result of observation. They are improved and extended as further observations are made. For some of these frames (such as spacecraft fixed frames) vary large data sets are needed to express the orientation of the frame with respect to other frames. Frame transformations that are a function of time and require megabytes of data are not suitable for encapsulation in C or FORTRAN source code. As a result, in the SPICE system, the computation of non-inertial frame transformations depends upon data stored in other SPICE kernels. If you request states relative to a non-inertial frame or use ephemerides that are represented relative to non-inertial frames you must load additional SPICE kernels. The method by which an auxiliary kernel is loaded depends upon the type of the kernel.
There are currently four classes of reference frames that are supported by the SPICE system. We give a brief overview of these frames here. For a more thorough discussion of the various types of frames see the recommended reading file ``FRAMES.REQ.''
Inertial frames
pcklof_c ( <file>, &handle ); {PCK Load
Orientation File}
pckuof_c ( <handle> ); {PCK Unload
Orientation File}
ldpool_c ( <file> ) {Load Kernel Pool}
CK Frames
cklpf_c ( <file>, &handle ); {CK Load Pointing File}
ckupf_c ( <handle> ); {CK Unload Pointing File}
ldpool_c ( <sclk_file_name> ); {Load Kernel Pool}
TK Frames
ldpool_c ( <TK_frame_file> ); {Load Kernel Pool}
In addition to the files mentioned above, it may be necessary to load a
``frame definition'' file along with the one of the SPICE kernels listed
above. (If the producer of the file has done his or her homework this step
should be unnecessary.) The frame definition file is a SPICE text kernel that
specifies the type of the frame, the center of the frame, the name of the
frame, and its ID code. (See FRAMES.REQ for more details concerning frame
definitions.)
As is evident from the above discussion, the use of non-inertial frames requires more data management on the part of the user of the SPICE system. However, this data management problem is not a new problem. In previous versions of the SPICE system the same kernels would have been required. Moreover, in previous versions of the SPICE system, you would have been required to perform all non-inertial transformations in your own code. With the inclusion of non-inertial frames in the SPK system, we have relieved you of some of the tasks associated with non-inertial frames.
An SPK file is made up of one or more data ``segments'' and a ``comment''
area. These components are described below.
An SPK file contains one or more ``segments.'' Each segment contains
ephemeris data sufficient to compute the geometric state (position and
velocity) of one solar system body (the `target') with respect to another
(the `center') at any epoch throughout some finite interval of time.
Either body may be a spacecraft, a planet or planet barycenter, a satellite, a comet, an asteroid, a tracking station, a roving vehicle, or an arbitrary point for which an ephemeris has been calculated. Each body in the solar system is associated with a unique integer code. A list of names and codes for the planets, major satellites, spacecraft, asteroids and comets can be found in the document NAIF_IDS.REQ
The states computed from the ephemeris data in a segment must be referenced to a single, recognized reference frame.
The data in each segment are stored as an array of double precision numbers. The summary for the array, called a `descriptor', has two double precision components:
Preceding the `segments', the Comment Area provides space in the SPK file for
storing textual information besides what is written in the array names.
Ideally, each SPK file would contain internal documentation that describes
the origin, recommended use, and any other pertinent information about the
data in that file. For example, the beginning and ending epochs for the file,
the names and NAIF integer codes of the bodies included, an accuracy
estimate, the date the file was produced, and the names of the source files
used in making the SPK file could be included in the Comment Area.
The utility programs COMMNT and SPACIT may be used to examine and manipulate the comments in an SPK file. In addition to these utilities, CSPICE provides a family of functions for handling this Comment Area. The name of each function in this family begins with the letters `SPC' which stand for `SPk and Ck' because this feature is common to both types of files. The SPC software provides the ability to add, extract, read, and delete comments and convert commented files from binary format to SPICE transfer format and back to binary again.
The SPC functions and their functions are described in detail in the SPC Required Reading.
The fourth integer component of the descriptor---the code for the
representation, or `data type'---is the key to the SPK format.
For purposes of determining the segment best suited to fulfill a particular request, all segments are treated equally. It is only when the data in a segment are to be evaluated---when a state vector is to be computed---that the type of data used to represent the ephemeris becomes important.
Because this step is isolated within a single low-level reader, spkpvn_c, new data types can be added to the SPK format without affecting application programs that use the higher level readers. spkpvn_c is designed so that the changes required to implement a new data type are minimal.
There are no real limits on the possible representations that can be used for ephemeris data. Users with access to data suitable for creating an ephemeris may choose to invent their own representations, adapting spkpvn_c accordingly. (We recommend that you consult with NAIF prior to implementing an new data type.)
The data types currently supported by CSPICE software are listed under ``Supported Data Types'' later in this document.
When computing states, spkezr_c should be sufficient to handle the needs of
most users. However, it is possible to exercise more direct control over the
way states are computed. In this section we discuss means by which the user
of the SPK system can take more direct control over the computation of
states.
As noted above, spkezr_c is identical to spkez_c except that it uses the names of objects as inputs instead of integer ID codes. Indeed, spkezr_c ``looks up'' the ID codes associated with the named objects, and then calls spkez_c using these ID codes.
spkez_c computes apparent and true states using two readers of slightly less power---spkssb_c and spkapp_c.
spkssb_c returns the state of a body with respect to the solar system barycenter. spkez_c uses it to compute the state of the observer relative to an inertial reference frame.
The second, spkapp_c, returns the state of a target body as seen from an observer. Where spkez_c requires the integer code for an observer, spkapp_c requires the actual state of the observer with respect to the solar system barycenter relative to an inertial reference frame. A single call to spkez_c,
spkez_c ( 801, et, "j2000", "lt+s", -32, starg, < ) { Easy
state }
is equivalent to a pair of calls to spkssb_c and spkapp_c:
spkssb_c ( -32,
et,
"j2000",
stobs ); { Solar system barycenter }
spkapp_c ( 801,
et,
"j2000",
stobs,
"lt+s",
starg,
< ); { Apparent state }
(It is important to note that this equivalence breaks down if spkez_c is
requested to return states relative to a non-inertial frame. When
non-inertial apparent or true states are requested, spkez_c first computes an
inertial apparent or true state. After the inertial state has been computed,
it is transformed to a non-inertial frame taking into account the light time
delay from the center of that frame.)
One possible advantage of using spkapp_c directly is the ability to place an observer somewhere other than at the center of a body (for example, at a specified location on the surface of the Earth).
When computing uncorrected, that is, geometric states, spkez_c does not need to compute the state of the target and observer relative to the solar system barycenter, but only relative to the first common center of motion of those two bodies. spkez_c calls spkgeo_c to compute geometric states.
Using spkgeo_c instead of the combination spkssb_c and spkapp_c as above prevents possible round-off error, may reduce the number of file reads, and may require less data. For example, if SPK ephemeris data for a spacecraft relative to a planet has been loaded, but the ephemeris data for that planet relative to the solar system barycenter is not available, spkgeo_c can still compute the state of the spacecraft relative to the planet, whereas the combination spkssb_c and spkapp_c would be unsuccessful at computing that state.
At the lowest level, it is possible to compute states without combining them
at all. Given the handle and descriptor for a particular segment, function
spkpvn_c returns a state from that segment directly.
spkpvn_c ( <handle>,
<descr>,
<et>,
ref,
state,
center ); { Position, velocity, native frame }
spkpvn_c is the most basic SPK reader. It returns states relative to the
frame in which they are stored in the SPK file. It does not rotate or combine
them: it returns a state relative to the center whose integer code is stored
in the descriptor for the segment. This state is relative to the frame whose
integer ID code is also stored in the descriptor of the segment. The user is
responsible for using that state correctly.
The user is also responsible for using DAF functions to determine the particular file and segment from which each state is to be computed.
Note that to use the state returned by spkpvn_c in any frame other than the ``native frame'' of the segment, you must convert the state to the frame of interest. A second low level function spkpv_c can be used to perform the state transformations for you. The calling sequence for spkpv_c is identical to that for spkpvn_c. However, in the case of spkpv_c the reference frame is an input instead of an output argument.
spkpv_c ( <handle>,
<descr>,
<et>,
<ref>,
state,
center ); { Position, velocity }
Thus using spkpv_c instead of spkpvn_c allows you to avoid the details of
converting states to the frame of interest.
If files have been loaded by previous calls to spklef_c, it is possible to use the same segments that would normally be used by spkezr_c, spkez_c, spkssb_c, spkapp_c, and spkgeo_c. Function spksfs_c selects, from the database of loaded files, the file handle and segment descriptor for the segment best suited to the request. If two segments from different files are suitable, spksfs_c selects the one from the file that was loaded later. If two segments from the same file are suitable, spksfs_c selects the one that is stored later in the file. The call
spksfs_c ( <801>,
<et>,
&handle,
descr,
segnam,
&found ); { Select file and segment }
returns the handle, descriptor, and segment name for the latest segment
containing data for Triton at the specified epoch. spksfs_c maintains a
buffer of segment descriptors and segment names, so it doesn't waste time
searching the database for bodies it already knows about.
The next several sections present sample programs to show how the SPK readers
can be used to compute state vectors, and how those vectors can be used to
compute derived quantities.
All functions used in the examples are from CSPICE. The convention of expanding SPK function names will be dropped for these examples.
The first example program computes the planetocentric latitude and longitude of the sub-observer point on a target body for any combination of observer, target, and epoch. (Note that planetocentric coordinates differ from planetographic and cartographic coordinates in that they are always right-handed, regardless of the rotation of the body. Also note that for this example we define the sub-observer point to be the point on the ``surface'' of the target that lies on the ray from the center of the target to the observer. )
/*
PROGRAM LATLON
*/
/*
Standard includes
*/
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
/*
CSPICE prototypes and definitions.
*/
#include "SpiceUsr.h"
void main()
{
/*
Constants
*/
#define NAMLEN 32
/*
Variables
*/
SpiceChar * time;
SpiceChar * obs;
SpiceChar * targ;
SpiceChar frame [NAMLEN];
SpiceDouble et;
SpiceDouble lat;
SpiceDouble lon;
SpiceDouble lt;
SpiceDouble radius;
SpiceDouble state [6];
SpiceInt h [13];
/*
Load constants into the kernel pool. Two files are
needed. The first ("time.ker") contains the dates
of leap seconds and values for constants needed to
compute the difference between UTC and ET at any
epoch. The second ("pck.ker") contains IAU values
needed to compute transformations from inertial
(J2000) coordinates to body-fixed (pole and prime
meridian) coordinates for the major bodies of the
solar system. (These files, or their equivalents,
are normally distributed along with CSPICE.)
*/
ldpool_c ( "time.ker" );
ldpool_c ( "pck.ker" );
/*
Several ephemeris files are used. Most contain data for
a single planetary system ("jupiter.ker", "saturn.ker",
and so on). Some contain data for spacecraft ("vgr1.ker",
"mgn.ker").
*/
spklef_c ( "mercury.bsp", &h[0] );
spklef_c ( "venus.bsp", &h[1] );
spklef_c ( "earth.bsp", &h[2] );
spklef_c ( "mars.bsp", &h[3] );
spklef_c ( "jupiter.bsp", &h[4] );
spklef_c ( "saturn.bsp", &h[5] );
spklef_c ( "uranus.bsp", &h[6] );
spklef_c ( "neptune.bsp", &h[7] );
spklef_c ( "pluto.bsp", &h[8] );
spklef_c ( "vgr1.bsp", &h[9] );
spklef_c ( "vgr2.bsp", &h[10] );
spklef_c ( "mgn.bsp", &h[11] );
spklef_c ( "gll.bsp", &h[12] );
/*
Inputs are entered interactively. The user enters three
items: the name for the observer , the name
for the target, and the UTC epoch at which the
sub-observer point is to be computed (a free-format string).
The epoch must be converted to ephemeris time (ET).
*/
while ( SPICETRUE )
{
obs = prompt_c ( "Observer? " );
targ = prompt_c ( "Target? " );
time = prompt_c ( "Epoch ? " );
str2et_c ( time, &et );
sprintf ( frame, "IAU_%s", targ );
/*
Compute the true state (corrected for light-time)
of the target as seen from the observer at the
specified epoch in the target fixed reference frame.
*/
spkezr_c ( targ, et, frame, "lt", obs, state, < );
/*
We need the vector FROM the target TO the observer
to compute latitude and longitude. So reverse it.
*/
vminus_c ( state, state );
/*
Convert from rectangular coordinates to latitude and
longitude, then from radians to degrees for output.
*/
reclat_c ( state, &radius, &lon, &lat );
printf ( "\n"
"Sub-observer latitude (deg): %f\n"
" longitude : %f\n"
"\n"
"Range to target (km) : %f\n"
"Light-time (sec) : %f\n"
"\n",
lat * dpr_c(),
lon * dpr_c(),
radius,
lt );
/*
Free the response strings allocated by prompt_c.
This step is required only to support extended
operation of the program.
*/
free ( obs );
free ( targ );
free ( time );
/*
Get the next set of inputs.
*/
}
}
The second example determines epochs if one target body (spacecraft, planet,
or satellite) is occulted by or in transit across another target body as seen
from an observer at a user specified epoch. It is similar in both form and
generality to the first example.
/*
PROGRAM OCCTRN
*/
/*
Standard includes
*/
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <math.h>
/*
CSPICE prototypes and definitions.
*/
#include "SpiceUsr.h"
void main()
{
/*
Constants
*/
#define NSPKS 2
#define NTARG 2
/*
Variables
*/
SpiceBoolean found;
SpiceChar * time;
SpiceChar * obs;
SpiceChar * targ [NTARG];
SpiceDouble avg;
SpiceDouble d [NTARG];
SpiceDouble et;
SpiceDouble lt;
SpiceDouble r [NTARG];
SpiceDouble radii [3];
SpiceDouble s [NTARG][6];
SpiceDouble sep;
SpiceInt dim;
SpiceInt i;
SpiceInt j;
SpiceInt k;
SpiceInt h [13];
SpiceInt t [NTARG];
/*
Load constants into the kernel pool. Two files are
needed. The first ("time.ker") contains the dates
of leap seconds and values for constants needed to
compute the difference between UTC and ET at any
epoch. The second ("radii.ker") contains values
for the tri-axial ellipsoids used to model the major
major bodies of the solar system.
*/
ldpool_c ( "time.ker" );
ldpool_c ( "radii.ker" );
/*
Several ephemeris files are needed. Most contain data for
a single planetary system ("jupiter.ker", "saturn.ker",
and so on). Some contain data for spacecraft ("vgr1.ker",
"mgn.ker").
*/
spklef_c ( "mercury.bsp", &h[0] );
spklef_c ( "gll.bsp", &h[NSPKS-1] );
/*
Inputs are entered interactively. The user enters four
items: the code for the observer (an integer), the codes
for two target bodies (integers), and the epoch at which
check for occultation or transit is to be computed
(a free-format string).
The epoch must be converted to ephemeris time (ET).
*/
while ( SPICETRUE )
{
obs = prompt_c ( "Observer? " );
targ[0] = prompt_c ( "Target 1? " );
targ[1] = prompt_c ( "Target 2? " );
time = prompt_c ( "Epoch? " );
str2et_c ( time, &et );
/*
Get the ID codes associated with the targets
*/
bodn2c_c ( targ[0], &t[0], &found );
bodn2c_c ( targ[1], &t[1], &found );
/*
Get the apparent states of the target objects as seen from
the observer. Also get the apparent radius of each object
from the kernel pool. (Use zero radius for any spacecraft;
use average radius for anything else.)
t[i] is the ID code of the i'th target, i = 0, 1.
s[i][0..5] is the apparent state of the i'th target.
d[i] is the apparent distance to the i'th target.
r[i] is the apparent radius of the i'th target.
Function vnorm_c returns the Euclidean norm (magnitude) of
a three-vector.
Function sumad_c returns the sum of the elements in a
double precision array.
*/
for ( i = 0; i < 2; i++ )
{
spkezr_c ( targ[i], et, "J2000", "LT+S",
obs, s[i], < );
d[i] = vnorm_c( s[i] );
if ( t[i] < 0 )
{
r[i] = 0.;
}
else
{
bodvar_c ( t[i], "RADII", &dim, radii );
avg = sumad_c ( radii, 3 ) / 3.0;
r[i] = asin ( avg / d[i] );
}
}
/*
Determine the separation between the two bodies. If the
separation between the centers is greater than the sum of
the apparent radii, then the target bodies are clear of
each other.
Function vsep_c returns the angle of separation between
two three-vectors.
*/
sep = vsep_c ( s[0], s[1] ) - ( r[0] + r[1] );
if ( sep > 0. )
{
printf ( "\nClear.\n" );
/*
Otherwise, the smaller body is either occulted or
in transit. We compare ranges to decide which.
*/
}
else
{
/*
Let index j indicate the target of smaller radius; let k
indicate the larger target.
*/
( r[0] < r[1] ) ? (j = 0) : (j = 1);
k = 1-j;
if ( d[j] < d[k] )
{
printf ( "\n%s is in transit across %s\n",
targ[j],
targ[k] );
}
else
{
printf ( "\n%s is occulted by %s\n",
targ[j],
targ[k] );
}
}
/*
Free the response strings allocated by prompt_c.
This step is required only to support extended
operation of the program.
*/
free ( obs );
free ( targ[0] );
free ( targ[1] );
free ( time );
/*
Get the next set of inputs.
*/
}
}
Additional, working examples of using the principal SPK functions may be
found in the ``Cookbook'' programs distributed with the NAIF Toolkit.
The following representations, or data types, are currently supported by the
SPK functions in CSPICE.
The first SPK data type contains Modified Difference Arrays (MDA), sometimes
called `difference lines'. This data type is normally used for spacecraft
whose ephemerides are produced by JPL's principal trajectory
integrator---DPTRAJ. Difference lines are extracted from the spacecraft
trajectory file (`P-files' and `PV-files') created by DPTRAJ.
Each segment containing Modified Difference Arrays contains an arbitrary number of logical records. Each record contains difference line coefficients valid up to some final epoch, along with the state at that epoch. The contents of the records themselves are described in [163]. The function spke01_ contains the algorithm used to construct a state from a particular record and epoch.
The records within a segment are ordered by increasing final epoch. A segment of this type is structured as follows:
+-----------------------------------------+
| Record 1 (difference line coefficients) |
+-----------------------------------------+
| Record 2 (difference line coefficients) |
+-----------------------------------------+
.
.
.
+-----------------------------------------+
| Record N (difference line coefficients) |
+-----------------------------------------+
| Epoch 1 |
+------------------------------+
| Epoch 2 |
+------------------------------+
.
.
.
+------------------------------+
| Epoch N |
+------------------------------+
| Directory epoch 1 | (First directory epoch)
+------------------------------+
| Directory epoch 2 |
+------------------------------+
.
.
.
+------------------------------+
| Directory epoch (N/100)*100 | (Final directory epoch)
+------------------------------+
| N |
+------------------------------+
The number of records in a segment, N, can be arbitrarily large.
Records 1 through N contain the difference line coefficients and other constants needed to compute state data. Each one of these records contains 71 double precision numbers.
The list of final epochs for the records is stored immediately after the last record.
Following the list of epochs is a second list, the `directory', containing every 100th epoch from the previous list. If there are N epochs, there will be N/100 directory epochs. If there are fewer than 100 epochs, then the segment will not contain any directory epochs. Directory epochs are used to speed up access to desired records.
The final element in the segment is the number of records contained in the segment, N.
The index of the record corresponding to a particular epoch is the index of the first epoch not less than the target epoch.
The second SPK data type contains Chebyshev polynomial coefficients for the
position of the body as a function of time. Normally, this data type is used
for planet barycenters, and for satellites whose ephemerides are integrated.
(The velocity of the body is obtained by differentiating the position.)
Each segment contains an arbitrary number of logical records. Each record contains a set of Chebyshev coefficients valid throughout an interval of fixed length. The function spke02_ contains the algorithm used to construct a state from a particular record and epoch.
The records within a segment are ordered by increasing initial epoch. All records contain the same number of coefficients. A segment of this type is structured as follows:
+---------------+
| Record 1 |
+---------------+
| Record 2 |
+---------------+
.
.
.
+---------------+
| Record N |
+---------------+
| INIT |
+---------------+
| INTLEN |
+---------------+
| RSIZE |
+---------------+
| N |
+---------------+
A four-number `directory' at the end of the segment contains the information
needed to determine the location of the record corresponding to a particular
epoch.
+------------------+ | MID | +------------------+ | RADIUS | +------------------+ | X coefficients | +------------------+ | Y coefficients | +------------------+ | Z coefficients | +------------------+The first two elements in the record, MID and RADIUS, are the midpoint and radius of the time interval covered by coefficients in the record. These are used as parameters to perform transformations between the domain of the record (from MID - RADIUS to MID + RADIUS) and the domain of Chebyshev polynomials (from -1 to 1 ).
The same number of coefficients is always used for each component, and all records are the same size (RSIZE), so the degree of each polynomial is
( RSIZE - 2 ) / 3 - 1To facilitate the creation of Type 2 segments, a segment writing function called spkw02_c has been provided. This function takes as input arguments the handle of an SPK file that is open for writing, the information needed to construct the segment descriptor, and the data to be stored in the segment. The header of the function provides a complete description of the input arguments and an example of its usage.
The third SPK data type contains Chebyshev polynomial coefficients for the
position and velocity of the body as a function of time. Normally, this data
type is used for satellites for which the ephemerides are computed from
analytical theories.
The structure of the segment is nearly identical to that of the SPK data Type 2 (Chebyshev polynomials for position only). The only difference is that each logical record contains six sets of coefficients instead of three. The function spke03_ contains the algorithm used to construct a state from a particular record and epoch.
Each record is structured as follows:
+------------------+ | MID | +------------------+ | RADIUS | +------------------+ | X coefficients | +------------------+ | Y coefficients | +------------------+ | Z coefficients | +------------------+ | X' coefficients | +------------------+ | Y' coefficients | +------------------+ | Z' coefficients | +------------------+The same number of coefficients is always used for each component, and all records are the same size (RSIZE), so the degree of each polynomial is
( RSIZE - 2 ) / 6 - 1To facilitate the creation of Type 3 segments, a segment writing function called spkw03_c has been provided. This function takes as input arguments the handle of an SPK file that is open for writing, the information needed to construct the segment descriptor, and the data to be stored in the segment. The header of the function provides a complete description of the input arguments and an example of its usage.
The fifth standard SPK data type contains discrete state vectors. A state is
obtained from a Type 5 segment for any epoch that is within the bounds of
that segment by propagating the discrete states to the specified epoch
according to the laws of two body motion. Normally, this data type is used
for comets and asteroids, whose ephemerides are integrated from an initial
state or set of osculating elements.
Each segment contains of a number of logical records. Each record consists of an epoch (ephemeris seconds past J2000) and the geometric state of the body at that epoch (x, y, z, dx/dt, dy/dt, dz/dt, in kilometers and kilometers per second). Records are ordered with respect to increasing time.
The records that correspond to an epoch for which a state is desired are the ones whose associated epochs bracket that epoch. The state in each record is used as the initial state in a two-body propagation; a weighted average of the propagated states gives the position of the body at the specified epoch. The velocity is given by the derivative of the position. Thus the position and velocity at the specified epoch are given by:
P = W(t) * P1(t) + (1-W(t)) * P2(t) V = W(t) * V1(t) + (1-W(t)) * V2(t) + W'(t) * ( P1(t) - P2(t) )where P1, V1, P2, and V2 are the position and velocity components of the propagated states and W is the weighting function.
The weighting function used is:
W(t) = 0.5 + 0.5 * cos [ PI * ( t - t1 ) / ( t2 - t1 ) ]where t1 and t2 are the epochs that bracket the specified epoch t.
Physically, the epochs and states are stored separately, so that the epochs can be searched as an ordered array. Thus, the initial part of each segment looks like this:
+--------------------+
| State 1 |
+--------------------+
.
.
.
+--------------------+
| State N |
+--------------------+
| Epoch 1 |
+--------------------+
.
.
.
+--------------------+
| Epoch N |
+--------------------+
The number of records in a segment can be arbitrarily large. In order to
avoid the file reads required to search through a large array of epochs, each
segment contains a simple directory immediately after the final epoch.
This directory contains every 100th epoch in the epoch array. If there are N epochs, there will be N/100 directory epochs. (If there are fewer than 100 epochs, no directory epochs are stored.)
The final items in the segment are GM, the gravitational parameter of the central body (kilometers and seconds), and N, the number of states in the segment. Thus, the complete segment looks like this:
+--------------------+
| State 1 |
+--------------------+
.
.
.
+--------------------+
| Epoch 1 |
+--------------------+
.
.
.
+--------------------+
| Epoch N |
+--------------------+
| Epoch 100 | (First directory epoch)
+--------------------+
| Epoch 200 |
+--------------------+
.
.
.
+--------------------+
| Epoch (N/100)*100 | (Final directory epoch)
+--------------------+
| GM |
+--------------------+
| N |
+--------------------+
To facilitate the creation of Type 5 segments, a segment writing function
called spkw05_c has been provided. This function takes as input arguments the
handle of an SPK file that is open for writing, the information needed to
construct the segment descriptor, and the data to be stored in the segment.
The header of the function provides a complete description of the input
arguments and an example of its usage.
The eighth SPK data type represents a continuous ephemeris using a discrete
set of states and a Lagrange interpolation method. The epochs (also called
`time tags') associated with the states must be evenly spaced: there must be
some positive constant STEP such that each time tag differs from its
predecessor and successor by STEP seconds. For a request epoch not
corresponding to the time tag of some state, the data type defines a state by
interpolating each component of a set of states whose epochs are `centered'
near the request epoch. Details of how these states are selected and
interpolated are given below.
The SPK system can also represent an ephemeris using unequally spaced discrete states and Lagrange interpolation; SPK Type 9 does this. SPK Type 9 sacrifices some run-time speed and economy of storage in order to achieve greater flexibility.
The states in a Type 8 segment are geometric: they do not take into account aberration corrections. The six components of each state vector represent the position and velocity (x, y, z, dx/dt, dy/dt, dz/dt, in kilometers and kilometers per second) of the body to which the ephemeris applies, relative to the center specified by the segment's descriptor. The epochs corresponding to the states are barycentric dynamical times (TDB), expressed as seconds past J2000.
Each segment also has a polynomial degree associated with it; this is the degree of the interpolating polynomials to be used in evaluating states based on the data in the segment. The identical degree is used for interpolation of each state component.
Type 8 SPK segments have the structure shown below:
+--------+
| x(1) |
/ +--------+
/ | y(1) |
/ +--------+
/ | z(1) |
+-----------------------+ / +--------+
| State 1 | < |dx(1)/dt|
+-----------------------+ \ +--------+
| State 2 | \ |dy(1)/dt|
+-----------------------+ \ +--------+
. \ |dz(1)/dt|
. +--------+
.
+-----------------------+
| State N |
+-----------------------+
| Epoch of state 1 (TDB)|
+-----------------------+
| Step size |
+-----------------------+
| Polynomial degree |
+-----------------------+
| Number of states |
+-----------------------+
In the diagram, each box representing a state vector corresponds to six
double precision numbers; the other boxes represent individual double
precision numbers. Since the epochs of the states are evenly spaced, they are
represented by a start epoch and a step size. The number of states must be
greater than the interpolating polynomial degree.
The Type 8 interpolation method works as follows: given an epoch at which a state is requested and a segment having coverage for that epoch, the Type 8 reader finds a group of states whose epochs are `centered' about the epoch. The size of the group is one greater than the polynomial degree associated with the segment. If the group size N is even, then the group will consist of N consecutive states such that the request time is between the epochs of the members of the group having indices, relative to the start of the group, of N/2 and (N/2 + 1), inclusive. When N is odd, the group will contain a central state whose epoch is closest to the request time, and will also contain (N-1)/2 neighboring states on either side of the central one. The Type 8 evaluator will then use Lagrange interpolation on each component of the states to produce a state corresponding to the request time. For the jth state component, the interpolation algorithm is mathematically equivalent to finding the unique polynomial of degree N-1 that interpolates the ordered pairs
( epoch(i), state(j,i) ), i = k , k , ... , k
1 2 N
and evaluating the polynomial at the requested epoch. Here
k , k , ... , k
1 2 N
are the indices of the states in the interpolation group,
epoch(i)is the epoch of the ith state and
state(j,i)is the jth component of the ith state.
There is an exception to the state selection algorithm described above: the request time may be too near the first or last state of the segment to be properly bracketed. In this case, the set of states selected for interpolation still has size N, and includes either the first or last state of the segment.
To facilitate the creation of Type 8 segments, a segment writing function called spkw08_c has been provided. This function takes as input arguments the handle of an SPK file that is open for writing, the information needed to construct the segment descriptor, and the data to be stored in the segment. The header of the function provides a complete description of the input arguments and an example of its usage.
The ninth SPK data type represents a continuous ephemeris using a discrete
set of states and a Lagrange interpolation method. The epochs (also called
`time tags') associated with the states need not be evenly spaced. For a
request epoch not corresponding to the time tag of some state, the data type
defines a state by interpolating each component of a set of states whose
epochs are `centered' near the request epoch. Details of how these states are
selected and interpolated are given below.
The states in a Type 9 segment are geometric: they do not take into account aberration corrections. The six components of each state vector represent the position and velocity (x, y, z, dx/dt, dy/dt, dz/dt, in kilometers and kilometers per second) of the body to which the ephemeris applies, relative to the center specified by the segment's descriptor. The epochs corresponding to the states are barycentric dynamical times (TDB), expressed as seconds past J2000.
Each segment also has a polynomial degree associated with it; this is the degree of the interpolating polynomials to be used in evaluating states based on the data in the segment. The identical degree is used for interpolation of each state component.
Type 9 SPK segments have the structure shown below:
+--------+
| x(1) |
/ +--------+
/ | y(1) |
/ +--------+
/ | z(1) |
+-----------------------+ / +--------+
| State 1 | < |dx(1)/dt|
+-----------------------+ \ +--------+
| State 2 | \ |dy(1)/dt|
+-----------------------+ \ +--------+
. \ |dz(1)/dt|
. +--------+
.
+-----------------------+
| State N |
+-----------------------+
| Epoch 1 |
+-----------------------+
| Epoch 2 |
+-----------------------+
.
.
.
+-----------------------+
| Epoch N |
+-----------------------+
| Epoch 100 | (First directory)
+-----------------------+
.
.
.
+-----------------------+
| Epoch ((N-1)/100)*100 | (Last directory)
+-----------------------+
| Polynomial degree |
+-----------------------+
| Number of states |
+-----------------------+
In the diagram, each box representing a state vector corresponds to six
double precision numbers; the other boxes represent individual double
precision numbers. The number of states must be greater than the
interpolating polynomial degree.
The set of time tags is augmented by a series of directory entries; these entries allow the Type 9 reader to search for states more efficiently. The directory entries contain time tags whose indices are multiples of 100. The set of indices of time tags stored in the directories ranges from 100 to
( (N-1) / 100 ) * 100where N is the total number of time tags. Note that if N is
Q * 100then only
Q - 1directory entries are stored, and in particular, if there are only 100 states in the segment, there are no directories.
The Type 9 interpolation algorithm is virtually identical to the Type 8 algorithm; see the discussion of SPK Type 8 for details. However, the Type 9 algorithm executes more slowly than the Type 8 algorithm, since the Type 9 reader must search through tables of time tags to find appropriates states to interpolate, while the Type 8 reader can locate the correct set of states to interpolate by a direct computation.
To facilitate the creation of Type 9 segments, a segment writing function called spkw09_c has been provided. This function takes as input arguments the handle of an SPK file that is open for writing, the information needed to construct the segment descriptor, and the data to be stored in the segment. The header of the function provides a complete description of the input arguments and an example of its usage.
The SPK data Type 10 uses the SPICE concept of a generic segment to store a
collection of packets each of which models the trajectory of some Earth
satellite using Space Command two-line elements (formerly the North American
Air Defense --- NORAD).
The storage, arrangement and retrieval of two-line element sets is handled by the SPICE generic segment software described in the document GENSEG.REQ. (The document GENSEG.REQ is currently in preparation.) We review only the pertinent points about generic segments here.
A generic SPK segment contains several logical data partitions:
+============================+
| Constants |
+============================+
| Packet 1 |
|----------------------------|
| Packet 2 |
|----------------------------|
| . |
| . |
| . |
|----------------------------|
| Packet N |
+============================+
| Reference Epochs |
+============================+
| Packet Directory |
+============================+
| Epoch Directory |
+============================+
| Reserved Area |
+============================+
| Segment Meta Data |
+----------------------------+
Only the placement of the meta data at the end of a generic segment is
required. The other data partitions may occur in any order in the generic
segment because the meta data will contain pointers to their appropriate
locations within the generic segment.
Each ``packet'' of a Type 10 segment contains one set of two-line elements, the nutations in longitude and obliquity of the Earth's pole, and the rates of these nutations. Each packet is arranged as shown below. (The notation below is taken from the description that accompanies the code available from Space Command for the evaluation of two-line elements.)
+-------------------+
1 | NDT20 |
+-------------------+
2 | NDD60 |
+-------------------+
3 | BSTAR |
+-------------------+
4 | INCL |
+-------------------+
5 | NODE0 | Two-line element packet
+-------------------+
6 | ECC |
+-------------------+
7 | OMEGA |
+-------------------+
8 | MO |
+-------------------+
9 | NO |
+-------------------+
10 | EPOCH |
+-------------------+
11 | NU.OBLIQUITY |
+-------------------+
12 | NU.LONGITUDE |
+-------------------+
13 | dOBLIQUITY/dt |
+-------------------+
14 | dLONGITUDE/dt |
+-------------------+
The constants partition of the Type 10 segment contains the following eight
constants.
+-------------------------------------------+
1 | J2 gravitational harmonic for Earth |
+-------------------------------------------+
2 | J3 gravitational harmonic for Earth |
+-------------------------------------------+
3 | J4 gravitational harmonic for Earth |
+-------------------------------------------+
| Square root of the GM for Earth where GM |
4 | is expressed in Earth radii cubed per |
| minutes squared |
+-------------------------------------------+
5 | Equatorial radius of the Earth in km |
+-------------------------------------------+
6 | Low altitude bound for atmospheric |
| model in km |
+-------------------------------------------+
7 | High altitude bound for atmospheric |
| model in km |
+-------------------------------------------+
8 | Distance units/Earth radius (normally 1) |
+-------------------------------------------+
The reference epochs partition contains an ordered collection of epochs. The
i'th reference epoch is equal to the epoch in the i'th packet.
The ``epoch directory'' contains every 100th reference epoch. The epoch directory is used to efficiently locate an the reference epoch that should be associated with a two line element packet.
The ``packet directory'' is empty.
As noted above the exact location of the various partitions must be obtained from the Meta data contained at the end of the segment. Access to the data should be made via the SPICELIB generic segment functions or via the SPK Type 10 reader---spkr10_. The function spkw10_c is available for writing a Type 10 generic segment.
The SPK data Type 14 uses the SPICE concept of a generic segment to store a
collection of packets each of which models the trajectory of some object with
respect to another over some interval of time. Each packet contains a set of
coefficients for Chebyshev polynomials that approximate the position and
velocity of some object. The time intervals corresponding to each packet are
non-overlapping. Moreover their union covers the interval of time spanned by
the start and end times of the Type 14 segment. Unlike Types 2 and 3 the time
spacing between sets of coefficients for a Type 14 segment may be
non-uniform.
The storage, arrangement and retrieval of packets is handled by the SPICE generic segment software. That software is documented in the document GENSEG.REQ. (The document GENSEG.REQ is currently in preparation.) We only review the pertinent points about generic segments here.
A generic SPK segment contains several logical data partitions:
+============================+
| Constants |
+============================+
| Packet 1 |
|----------------------------|
| Packet 2 |
|----------------------------|
| . |
| . |
| . |
|----------------------------|
| Packet N |
+============================+
| Reference Epochs |
+============================+
| Packet Directory |
+============================+
| Epoch Directory |
+============================+
| Reserved Area |
+============================+
| Segment Meta Data |
+----------------------------+
Only the placement of the meta data at the end of a generic segment is
required. The other data partitions may occur in any order in the generic
segment because the meta data will contain pointers to their appropriate
locations within the generic segment.
In the case of Type 14 SPK segments each ``packet'' contains an epoch, EPOCH, an allowed time offset, OFFSET, from the epoch, and 6 sets of Chebyshev polynomial coefficients which are used to evaluate the x,y,z, dx/dt, dy/dt, and dz/dt components of the state for epochs within OFFSET seconds of the EPOCH. Each packet is organized with the following structure:
------------------------------------------------ | The midpoint of the approximation interval | ------------------------------------------------ | The radius of the approximation interval | ------------------------------------------------ | CHBDEG+1 coefficients for the X coordinate | ------------------------------------------------ | CHBDEG+1 coefficients for the Y coordinate | ------------------------------------------------ | CHBDEG+1 coefficients for the Z coordinate | ------------------------------------------------ | CHBDEG+1 coefficients for the X velocity | ------------------------------------------------ | CHBDEG+1 coefficients for the Y velocity | ------------------------------------------------ | CHBDEG+1 coefficients for the Z velocity | ------------------------------------------------The maximum degree Chebyshev representation that can currently be accommodated is 18. Packets are stored in increasing order of the midpoint of the approximation interval.
The ``constants'' partition contains a single value, the degree of the Chebyshev representation.
The reference epochs partition contains an ordered collection of epochs. The i'th reference epoch corresponds to the beginning of the interval for which the i'th packet can be used to determine the state of the object modelled by this segment.
The ``epoch directory'' contains every 100th reference epoch. The epoch directory is used to efficiently locate an the reference epoch that should be associated with an epoch for which a state has been requested.
The ``packet directory'' is empty.
As noted above the exact location of the various partitions must be obtained from the Meta data contained at the end of the segment.
Access to the data should be made via the SPICELIB generic segment functions.
Type 14 segments should be created using the functions spk14b_c, spk14a_c, and spk14e_c. The usage of these functions is discussed in spk14b_c.
The SPK data Type 15 represents a continuous ephemeris using a compact
analytic model. The object is modelled as orbiting a central body under the
influence of a central mass plus first order secular effects of the J2 term
in harmonic expansion of the the central body gravitational potential.
Type 15 SPK segments have the structure shown below:
+--------------------------------+
| Epoch of Periapsis |
+--------------------------------+
| Trajectory pole_x |
+--------------------------------+
| Trajectory pole_y |
+--------------------------------+
| Trajectory pole_z |
+--------------------------------+
| Periapsis Unit Vector_x |
+--------------------------------+
| Periapsis Unit Vector_y |
+--------------------------------+
| Periapsis Unit Vector_z |
+--------------------------------+
| Semi-Latus Rectum |
+--------------------------------+
| Eccentricity |
+--------------------------------+
| J2 Processing Flag |
+--------------------------------+
| Central Body Pole_x |
+--------------------------------+
| Central Body Pole_y |
+--------------------------------+
| Central Body Pole_z |
+--------------------------------+
| Central Body GM |
+--------------------------------+
| Central Body J2 |
+--------------------------------+
| Central Body Equatorial Radius |
+--------------------------------+
It is important to note that the epoch must be that of periapsis passage.
Precession of the line of apsides and regression of the line of nodes is
computed relative to this epoch.
The effects of the J2 term are not applied if the eccentricity is greater than or equal to 1.
To facilitate the creation of Type 15 segments, a segment writing function called spkw15_c has been provided. This function takes as input arguments the handle of an SPK file that is open for writing, the information needed to construct the segment descriptor, and the data to be stored in the segment. The header of the function provides a complete description of the input arguments and an example of its usage.
The SPK data Type 17 represents a continuous ephemeris using a compact
analytic model. The object is following an elliptic orbit with precessing
line of nodes and argument of periapse relative to the equatorial frame of
some central body. The orbit is modelled via equinoctial elements.
Type 17 SPK segments have the structure shown below:
+----------------------------------+
1 | Epoch of Periapsis |
+----------------------------------+
2 | Semi-Major Axis |
+----------------------------------+
3 | H term of equinoctial elements |
+----------------------------------+
4 | K term of equinoctial elements |
+----------------------------------+
5 | Mean longitude at epoch |
+----------------------------------+
6 | P term of equinoctial elements |
+----------------------------------+
7 | Q term of equinoctial elements |
+----------------------------------+
8 | rate of longitude of periapse |
+----------------------------------+
9 | mean longitude rate |
+----------------------------------+
10 | longitude of ascending node rate |
+----------------------------------+
11 | equatorial pole right ascension |
+----------------------------------+
12 | equatorial pole declination |
+----------------------------------+
To facilitate the creation of Type 17 segments, a segment writing function
called spkw17_c has been provided. This function takes as input arguments the
handle of an SPK file that is open for writing, the information needed to
construct the segment descriptor, and the data to be stored in the segment.
The header of the function provides a complete description of the input
arguments and an example of its usage.
CSPICE contains a family of functions that are designed specifically for use
with SPK files. The name of each function begins with the letters `spk',
followed by a two- or three-character mnemonic. For example, the function
that returns the state of one body with respect to another is named spkez_c,
pronounced `S-P-K-E-Z'.
Many of the lower-level CSPICE functions have SPICELIB counterparts implemented in Fortran as entry points of another function.
The following is a complete list of mnemonics and translations, in alphabetical order.
Implemented CSPICE wrappers: spk14a_c ( S/P-kernel, add to a Type 14 segment ) spk14b_c ( S/P-kernel, begin a Type 14 segment ) spk14e_c ( S/P-kernel, end a Type 14 segment ) spkapo_c ( S/P-Kernel, "apparent" position only ) spkapp_c ( S/P-kernel, Apparent state ) spkcls_c ( S/P-kernel, close after write ) spkez_c ( S/P-kernel, Easy state ) spkezp_c ( S/P Kernel, easy position ) spkezr_c ( S/P-kernel, Easier state ) spkgeo_c ( S/P-kernel, Geometric state ) spkgps_c ( S/P Kernel, geometric position ) spklef_c ( S/P-kernel, Load ephemeris file ) spkopa_c ( S/P-kernel, open for addition ) spkopn_c ( S/P-kernel, open new file ) spkpds_c ( S/P-kernel, pack descriptor ) spkpos_c ( S/P Kernel, position ) spkssb_c ( S/P-kernel, Solar system barycenter ) spksub_c ( S/P-kernel, Subset a segment ) spkuds_c ( S/P-kernel, unpack descriptor ) spkuef_c ( S/P-kernel, Unload ephemeris file ) spkw02_c ( S/P-kernel, Write segment, Type 02 ) spkw03_c ( S/P-kernel, Write segment, Type 03 ) spkw05_c ( S/P-kernel, Write segment, Type 05 ) spkw08_c ( S/P-kernel, Write segment, Type 08 ) spkw09_c ( S/P-kernel, Write segment, Type 09 ) spkw10_c ( S/P-kernel, Write segment, Type 10 ) spkw15_c ( S/P-kernel, Write segment, Type 15 ) spkw17_c ( S/P-kernel, Write segment, Type 17 ) Unimplemented CSPICE wrappers: spkpv_c ( S/P-kernel, Position, velocity ) spkpvn_c ( S/P-kernel, Position, velocity---native) spksfs_c ( S/P-kernel, file and segment ) Low-level routines converted via f2c: spke01_ ( S/P-kernel, Evaluate record, Type 01 ) spke02_ ( S/P-kernel, Evaluate record, Type 02 ) spke03_ ( S/P-kernel, Evaluate record, Type 03 ) spke05_ ( S/P-kernel, Evaluate record, Type 05 ) spke08_ ( S/P-kernel, Evaluate record, Type 08 ) spke09_ ( S/P-kernel, Evaluate record, Type 09 ) spke10_ ( S/P-kernel, Evaluate record, Type 10 ) spke14_ ( S/P-kernel, Evaluate record, Type 14 ) spke15_ ( S/P-kernel, Evaluate record, Type 15 ) spke17_ ( S/P-kernel, Evaluate record, Type 17 ) spkr01_ ( S/P-kernel, Read record, Type 01 ) spkr02_ ( S/P-kernel, Read record, Type 02 ) spkr03_ ( S/P-kernel, Read record, Type 03 ) spkr05_ ( S/P-kernel, Read record, Type 05 ) spkr08_ ( S/P-kernel, Read record, Type 08 ) spkr09_ ( S/P-kernel, Read record, Type 09 ) spkr10_ ( S/P-kernel, Read record, Type 10 ) spkr14_ ( S/P-kernel, Read record, Type 14 ) spkr15_ ( S/P-kernel, Read record, Type 15 ) spkr17_ ( S/P-kernel, Read record, Type 17 ) spks01_ ( S/P-kernel, Subset data, Type 01 ) spks02_ ( S/P-kernel, Subset data, Type 02 ) spks03_ ( S/P-kernel, Subset data, Type 03 ) spks05_ ( S/P-kernel, Subset data, Type 05 ) spks08_ ( S/P-kernel, Subset data, Type 08 ) spks09_ ( S/P-kernel, Subset data, Type 09 ) spks10_ ( S/P-kernel, Subset data, Type 10 ) spks14_ ( S/P-kernel, Subset data, Type 14 ) spks15_ ( S/P-kernel, Subset data, Type 15 ) spks17_ ( S/P-kernel, Subset data, Type 17 )
The calling sequences for the SPK functions are summarized below. The
functions are grouped by purpose.
Loading, unloading files:
spklef_c ( fname, &handle ) spkuef_c ( handle )Computing states and positions:
spkezr_c ( tname, et, ref, aberr, oname, state, < ) spkpos_c ( tname, et, ref, aberr, oname, postn, < ) spkez_c ( target, et, ref, aberr, obs, state, < ) spkezp_c ( target, et, ref, aberr, obs, postn, < ) spkapp_c ( target, et, ref, stobs, aberr, state, < ) spkapo_c ( target, et, ref, stobs, aberr, postn, < ) spkssb_c ( target, et, ref, state ) spkgeo_c ( target, et, ref, obs, state, < ) spkgps_c ( target, et, ref, obs, postn, < ) Unimplemented: spkpvn_c ( handle, descr, et, &ref, state, ¢er ) spkpv_c ( handle, descr, et, ref, state, ¢er )Selecting files, segments:
Unimplemented: spksfs_c ( target, et, &handle, descr, pdgree, &found )Reading, evaluating records:
spkr01_ ( handle, descr, et, record ) spke01_ ( et, record, state ) spkr02_ ( handle, descr, et, record ) spke02_ ( et, record, state ) spkr03_ ( handle, descr, et, record ) spke03_ ( et, record, state ) spkr05_ ( handle, descr, et, record ) spke05_ ( et, record, state ) spkr08_ ( handle, descr, et, record ) spke08_ ( et, record, state ) spkr09_ ( handle, descr, et, record ) spke09_ ( et, record, state ) spkr10_ ( handle, descr, et, record ) spke10_ ( et, record, state ) spkr14_ ( handle, descr, et, record ) spke14_ ( et, record, state ) spkr15_ ( handle, descr, et, record ) spke15_ ( et, record, state ) spkr17_ ( handle, descr, et, record ) spke17_ ( et, record, state )Writing segments to files.
spkpds_c ( body, center, frame, type, first, last, descr )
spkw02_c ( handle, body, center, frame, first, last, segid,
intlen, n, polydg, cdata, btime )
spkw03_c ( handle, body, center, frame, first, last, segid,
intlen, n, polydg, cdata, btime )
spkw05_c ( handle, body, center, frame, first, last, segid,
gm, n, states, epochs )
spkw08_c ( handle, body, center, frame, first, last, segid,
degree, n, states, epoch1, step )
spkw09_c ( handle, body, center, frame, first, last, segid,
degree, n, states, epochs )
spkw10_c ( handle, body, center, frame, first, last,
segid, consts, n, elems, epochs )
spk14b_c ( handle, segid, body, center, frame,
first, last, chbdeg )
spk14a_c ( handle, ncsets, coeffs, epochs )
spk14e_c ( handle )
spkw15_c ( handle, body, center, frame, first, last, segid,
epoch, tpole, peri, p, ecc, j2flg, cpole,
gm, j2, radius )
spkw17_c ( handle, body, center, frame, first, last,
segid, epoch, epoch, eqel, rapol, decpol )
Examining segment descriptors:
spkuds_c ( descr, &body, ¢er, frame, &type,
&first, &last, &begin, &end )
Extracting subsets of data from a segment:
spks01_ ( handle, baddr, eaddr, begin, end ) spks02_ ( handle, baddr, eaddr, begin, end ) spks03_ ( handle, baddr, eaddr, begin, end ) spks05_ ( handle, baddr, eaddr, begin, end ) spks08_ ( handle, baddr, eaddr, begin, end ) spks09_ ( handle, baddr, eaddr, begin, end ) spks10_ ( handle, baddr, eaddr, begin, end ) spks14_ ( handle, baddr, eaddr, begin, end ) spks15_ ( handle, baddr, eaddr, begin, end ) spks17_ ( handle, baddr, eaddr, begin, end ) spksub_c ( handle, descr, ident, begin, end, newh )To write new or append segments to SPK files:
spkopn_c ( name, ifname, ncomch, handle ) spkopa_c ( file, handle ) spkcls_c ( handle )
An undocumented ephemeris is in many respects worse than undocumented source
code. With source code you can at least read the code and perhaps discern the
function of the source code. An ephemeris on the other hand is a binary file.
All it contains are numbers. It's very difficult to determine the purpose of
an ephemeris simply from the state information it contains. For this reason,
any ephemeris created for use by anyone other than yourself needs
documentation.
If you create SPK files NAIF strongly recommends that you include descriptive documentation in the comments portion of the SPK file. You can use the utility program COMMNT to insert comments into the file, or you may use the functions in the SPC family to insert the comments when you create the SPK file. (See COMMNT.UG or SPC.REQ for further details.)
This appendix addresses the contents of your comments. What will others (or yourself) want to know about the SPK file weeks, months or years after it has been created? Providing this information can be a challenge. It's difficult to know in advance all the questions someone might ask about an ephemeris you've created. To assist with this task NAIF has devised a ``template'' that you may wish to use as a starting point when creating the comments for an SPK file.
The comments you place in an SPK file must be plain ASCII text. Each line of
text must consist of 80 or fewer characters. The text must contain only
printing characters (ASCII characters 32 through 126).
Here's one way to create the comments for an SPK file.
List the names and NAIF ID codes for the objects in the file.
Provide a summary of the time for which states are available for the objects
in the file. If you use UTC times in this summary and the ephemeris extends
more than 6 months into the future, you should probably state that the times
are approximate. You don't know when leapseconds will occur more than a few
months in advance, so you can't know the exact UTC time boundaries for the
ephemeris if it extends years into the future.
Provide the ``status'' of the ephemeris. Tell the user why this ephemeris was
created and for whom it is intended. For example, if this is the second in a
series of ephemerides that will be produced for some object tell which
ephemeris this one supersedes. Tell the user when the next ephemeris in the
series will be available. Is the ephemeris suitable only for preliminary
studies? Is it good for all Earth based observations? Is this an official
operational product? Are there situations for which the ephemeris is not
suitable?
Provide a production summary for the ephemeris. Tell when the ephemeris was
produced (the system time stamp may not port if the file is copied to other
systems). Say who produced the ephemeris; what source products were used in
the production; what version of the producing program was used in the
creation of the ephemeris. If the ephemeris is based on a set of recent
observations, say so. In short give the user the pedigree of this ephemeris.
This information is mostly for your benefit. If a problem arises with the
ephemeris, you will know how the problem was created and have a better chance
of fixing the problem.
Provide information the user will need to effectively use the ephemeris. Tell
the user what other SPICE kernels are needed to use this ephemeris. For
example, if the ephemeris contains only the state of an asteroid relative to
the sun, the user will probably need a planetary ephemeris to effectively use
the one you've created. Recommend a planetary ephemeris to use with your SPK
file. If the ephemeris contains states of objects relative to non-inertial
frames, the user will probably need other kernels so that various state
transformations can be performed. Recommend which of these kernels the user
should use with your SPK file.
If possible give some estimate as to the accuracy of your SPK file. Use
numbers. Words such as ``this is the best available'' do not convey how much
you know about the ephemeris.
Provide a description of any special properties of this ephemeris. For
example, if some observation seems to be in conflict with this ephemeris you
should probably point this out.
List any references that may be relevant to the understanding of the
ephemeris. For example, if the ephemeris is based upon observations contained
in the literature, site the appropriate articles. If there is some technical
memorandum or private communication that addresses certain aspects of this
ephemeris list it. This will allow you to more easily answer questions about
the ephemeris.
List your phone number, mail or e-mail address so that users of the ephemeris
will be able to get in touch with you to ask questions or offer praise.